Light scattering from large molecules

FIGURE 26.5 Nephelometer. Light scattered by large molecules is measured at an angle a away from the axis of incident fight. The filters select the wavelength range desired and block undesired fluorescence. When a = 0, the technique is known as turbidimetry. 

In Section 8.7 a calculation was presented for a very stiff rod-like molecule. Although this model is adequate to describe light scattering from many real systems, most molecules have some degree of flexibility. When the intramolecular motions have large configurational changes associated with them, relaxation times for these motions will be... 

Light interacts primarily with the peripheral electrons in atoms and molecules and for this reason the nature of the interaction depends to a large extent on the chemical nature of the system." The extent to which materials refract and absorb light determines which systems can be analysed using the approach described here. An excellent review of what is known about light scattering from fractal aggregates has been published [26], and the interested reader should refer there for a much more detailed discussion. 

Further work with application to large molecules or particles in solution was done by Smoluchowski (J), Einstein (4), Debye (5, 6) and Zimm (7). For many years now the measurement of the intensity of the light scattered from dilute solutions of macromolecules as a function of concentration, and scattering angle has provided much important information on the size, shape, and thermodynamic properties of these... 

One of the most important fiinctions in the application of light scattering is the ability to estimate the object dimensions. As we have discussed earlier for dilute solutions containing large molecules, equation (B 1.9.38) can be used to calculate tire radius of gyration , R, which is defined as the mean square distance from the centre of gravity [12]. The combined use of equation (B 1.9.3 8) equation (B 1.9.39) and equation (B 1.9.40) (tlie Zimm plot) will yield infonnation on R, A2 and molecular weight. 

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